CODE:

//MyDLL.h
#pragma once

#define DLL_EXPORT __declspec(dllexport)

#include 

extern "C" {
INT DLL_EXPORT Add(INT x, INT y);
INT DLL_EXPORT Sub(INT x, INT y);
INT DLL_EXPORT Mul(INT x, INT y);
INT DLL_EXPORT Div(INT x, INT y);
}

CODE:

//MyDLL.cpp
#include "MyDLL.h"

INT Add(INT x, INT y){
	return x + y;
}

INT Sub(INT x, INT y){
	return x - y;
}

INT Mul(INT x, INT y){
	return x * y;
}

INT Div(INT x, INT y){
	return x / y;
}

CODE:

//MyDLLCall.cs
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using MMFrame.Diagnostics;

namespace DllCallTest
{

    public class MyDll : UnManagedDll
    {
        private class Functions
        {
            [UnmanagedFunctionPointer(CallingConvention.Cdecl)] public delegate int Add(int x, int y);
            [UnmanagedFunctionPointer(CallingConvention.Cdecl)] public delegate int Sub(int x, int y);
            [UnmanagedFunctionPointer(CallingConvention.Cdecl)] public delegate int Mul(int x, int y);
            [UnmanagedFunctionPointer(CallingConvention.Cdecl)] public delegate int Div(int x, int y);
        }

        public MyDll(string dllPath) : base(dllPath) { }

        public int Add(int x, int y)
        {
            return GetProcAddress()(x, y);
        }

        public int Sub(int x, int y)
        {
            return GetProcAddress()(x, y);
        }

        public int Mul(int x, int y)
        {
            return GetProcAddress()(x, y);
        }

        public int Div(int x, int y)
        {
            return GetProcAddress()(x, y);
        }
    }
}

//Program.cs
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace DllCallTest
{
    class Program
    {
        static void Main(string[] args)
        {
            try
            {
                MyDll dll = new MyDll("MyDLL.dll");
                Console.WriteLine("{0}", dll.Add(10, 9)); //19
                Console.WriteLine("{0}", dll.Sub(10, 9)); //1
                Console.WriteLine("{0}", dll.Mul(10, 9)); //90
                Console.WriteLine("{0}", dll.Div(10, 9)); //1
            }
            catch (Exception ex)
            {
                Console.WriteLine("{0}", ex.ToString());
            }
            Console.ReadKey();
        }
    }
}